The present invention relates to an off-line switched mode control system with fault condition protection.
Quantum leaps in electronic technology have led to the development of xe2x80x9csmartxe2x80x9d electrical and electronic products. Each of these products requires a steady and clean source of power from a power supply. One power supply technology called switched mode power supply technology operates at a high frequency to achieve small size and high efficiency. In such a switching power supply, an integrated circuit (IC) regulator is connected in series with the primary winding of a transformer to a rectified and filtered alternating current (AC) power line. The energy is transferred from the primary winding through an output secondary winding to the power supply output in a manner controlled by the IC regulator so as to provide a clean and constant output voltage. Additionally, a third winding called a feedback or bias winding may be used to provide a feedback signal and power to the IC regulator.
The voltage on the feedback winding tracks the output voltage present on the secondary winding. Thus, when a short occurs on the output of the secondary winding, the voltage on the feedback winding also goes low. Further, in the event of a short circuit condition, an overload condition on the output secondary winding or an open loop condition on the feedback winding, the regulator circuit responds to such conditions by delivering maximum power over a period of time. In such cases, the regulator circuit detects that the power supply is short circuited, overloaded at the output or has encountered an open loop condition. In any of these fault conditions, the regulator circuit goes into a mode called xe2x80x9cauto-restart.xe2x80x9d In the auto-restart mode, the regulator circuit tries to start the power supply periodically by delivering full power for a period of time (greater than needed for start up) and turns off the power supply for another period of time that is approximately four to ten times longer. As long as the fault condition is present, the regulator circuit remains in this auto-restart mode limiting the average output power to a safe, low value. When the fault is removed, auto-restart enables the power supply to start-up automatically.
The invention protects a power supply from fault conditions. The power supply has an output and a feedback control loop, the feedback control loop having a feedback signal which cycles periodically when the power supply operates normally and which remains idle when the power supply is in a fault condition. In a first aspect, the circuit includes a switching device for controlling power delivered to the output and a timer coupled to the switching device and to the feedback signal. The timer disables the switching device to prevent power delivery to the output in a first predetermined period after the fault condition exists.
Implementations of the invention include one or more of the following. The timer may enable the switching device to deliver power to the output after a second predetermined period. The switching device may be alternately enabled for the first predetermined period and disabled for the second predetermined period when the fault condition exists. The switching device may be enabled upon removal of the fault condition. The switching device may be a power transistor. The timer may be a digital counter. An oscillator with a predetermined frequency may be coupled to the counter. The oscillator may have a control input for changing the predetermined frequency and a first current source coupled to the oscillator control input to generate a first frequency. A second current source may be coupled to the oscillator control input to generate a second frequency. The counter""s output may be coupled to the fist and second current sources. The timer may be a capacitor which is adapted to be charged at a first rate from a first threshold to a second threshold to generate a first predetermined period. The capacitor may be discharged from the second threshold to the first threshold at a second rate to generate the second predetermined period. The capacitor may also be reset to a voltage below the first threshold each time the feedback signal cycles. The fault condition includes one or more of an output overload fault condition, an output short circuit fault condition and an open feedback control loop fault condition.
In a second aspect, a method for protecting a power supply having an output and a feedback control loop from fault conditions includes receiving a feedback signal from the feedback control loop, the feedback signal being adapted to cycle periodically when the power supply operates normally and to remain idle when the power supply is in a fault condition; timing the feedback signal to detect whether a fault condition exists in the power supply; and disabling the output after a first predetermined period after the fault condition is detected.
Implementations of the invention include one or more of the following. A switching device may be enabled to deliver power to the output after a second predetermined period. The switching device may be alternatingly enabled for the first predetermined period and disabled for the second predetermined period. The switching device may be enabled upon removal of the fault condition. The enabling step may enable a power transistor. The timing step includes digitally countering periods of time. A signal may be generated with a predetermined frequency. The generating step includes oscillating at a first frequency and a second frequency. The second frequency may be used when the fault condition exists. The timing step includes charging a capacitor at a first rate from a first threshold to a second threshold to generate a first predetermined period; and discharging the capacitor from the second threshold to the first threshold at a second rate to generate a second predetermined period. The capacitor may be reset to a voltage below the first threshold each time the feedback signal cycles.
In a third aspect, a circuit for protecting a power supply having an output and a feedback control loop from fault conditions includes means for receiving a feedback signal from the feedback control loop, the feedback signal being adapted to cycle periodically when the power supply operates normally and to remain idle when the power supply is in a fault condition; timing means coupled to the feedback signal to detect whether a fault condition exists in the power supply system; and means for disabling the output after a first predetermined period after the fault condition is detected.
Implementations of the invention include one or more of the following. The circuit includes a means for enabling a switching device to deliver power to the output after a second predetermined period. A means for alternatingly enabling the switching device for the first predetermined period and disabling the switching device for the second predetermined period when the fault condition exists may be used. The circuit may have a means for enabling the switching device upon removal of the fault condition. The switching device may be a power transistor. The timing means includes a digital counter. The circuit includes means for generating a predetermined frequency. The generating means includes means for oscillating at a first frequency and a second frequency. The circuit may include a means for applying the second frequency when the fault condition exists. The timing means includes a means for charging a capacitor at a first rate from a first threshold to a second threshold to generate a first predetermined period; and a means for discharging the capacitor from the second threshold to the first threshold at a second rate to generate a second predetermined period. A means for resetting the capacitor to a voltage below the first threshold each time the feedback signal cycles may be used.
In another aspect, a fault protected power supply includes a regulator coupled to a transformer having a primary winding. The transformer has a secondary winding coupled to a secondary output. The regulator receives a feedback signal from the secondary output which cycles periodically when the power supply operates normally and which remains idle when the power supply is in a fault condition. The power supply includes a switching device coupled to the primary winding of the transformer for controlling power delivered to the secondary output; an oscillator for generating a signal with a predetermined frequency; and a timer coupled to the oscillator and to the feedback signal, the timer disabling the switching device after a predetermined period of existence of a fault condition.
Implementations of the invention include one or more of the following. The power supply includes a means for changing the frequency of the oscillator. The timer alternatively enables and disables the switching means when the fault condition is present.
In another aspect, a method protects a power supply having a regulator coupled to a transformer having primary winding, the transformer having a secondary winding coupled to a secondary output, the regulator receiving a feedback signal from the secondary output which cycles periodically when the power supply operates normally and which remains idle when the power supply is in a fault condition. The method includes controlling power delivered to the secondary output using a switching device; generating an oscillating signal with a predetermined frequency; and timing the feedback signal with the oscillating signal and disabling the switching device after a predetermined period of existence of a fault condition.
Implementations of the invention include one or more of the following. The method includes changing the frequency of the oscillating signal. The method also includes alternatingly enabling and disabling the switching device when the fault condition is present.
In another aspect, a fault protected power supply has a regulator coupled to a transformer having a primary winding, the transformer having a secondary winding coupled to the secondary output. The regulator receives a feedback signal from the secondary output which cycles periodically when the power supply operates normally and which remains idle when the power supply is in a fault condition. The power supply includes a switching device coupled to the primary winding of the transformer for controlling the power delivered to the secondary output; a capacitor; means for charging the capacitor at a first rate from a first threshold to a second threshold to generate a first predetermined period and discharging the capacitor from the second threshold to first threshold at a second rate to generate a second predetermined period; and means coupled to the switching device, the capacitor and the feedback signal for alternately enabling the switching device during first predetermined period and disabling the switching device during the second predetermined period in the presence of a fault condition.
In yet another aspect, a method protects a power supply having a regulator coupled to a transformer having a primary winding. The transformer has a secondary winding coupled to a secondary output. The regulator receives a feedback signal from the secondary output which cycles periodically when the power supply operates normally and which remains idle when the power supply is in a fault condition. The method includes controlling power delivered to the secondary output using a switching device; charging a capacitor at a first rate from a first threshold to a second threshold to generate a first predetermined period; discharging the capacitor from the second threshold to first threshold at a second rate to generate a second predetermined period; and alternatingly enabling the switching device during the first predetermined period and disabling the switching device during the second predetermined period in the presence of a fault condition.
Advantages of the invention include one or more of the following. The invention protects the switched mode controller and associated components such as the diode and the transformer from various fault conditions. The feedback winding is not necessary. The protection is provided using a minimum number of components. Further, the power supply properly shuts down when it encounters a fault condition and automatically returns to an operating condition when the fault condition is removed.